Indirect ophthalmoscopy techniques are now in widespread use in diagnostic and therapeutic procedures in the field of ophthalmology. Indirect ophthalmoscopy techniques include the use of a hand-held lens, in conjunction with a binocular indirect ophthalmoscope, and more recently in biomicroscopic examination of the fundus using a slit lamp biomicroscope. The so-called hand-held condensing lens used in indirect ophthalmoscopy performs two functions: condensing the light from the source toward the entrance pupil of the eye, thereby illuminating the fundus, and forming an inverted real aerial image of the fundus at approximately the front focal distance of the lens. It has been found that indirect ophthalmoscopy is superior to direct ophthalmoscopy in the examination of retinopathies, retinal separation, retinal tumors, intraocular foreign bodies, and further in the ability to see fundus lesions which may not be viewable if there are opacities of the ocular media. The hand-held lenses used in indirect ophthalmoscopy have been of a variety of types, with each affording some advantages in the examination of the fundus.
The first hand-held indirect ophthalmoscopy lens which was used as a condensing and image-forming lens, included convex spherical surfaces and was of low power. The aerial image produced with such a spherical lens was magnified and inverted, but was quite blurred, particularly toward the periphery of the formed image. Subsequently, improvements were noted by the use of slightly higher powered lenses, each having one aspheric surface with the other surface being plano or spherical. Although the use of an aspherical surface in the indirect ophthalmoscopy lenses does show great improvement over spherical indirect ophthalmoscopy lenses, lens aberrations may remain such that light from the light source is not converged to a sharply defined image at the entrance pupil of the eye, and the formed aerial image of the fundus may show aberrations and increasing astigmatic effects particularly toward the periphery of the formed image. These designs were subsequently improved upon with the use of two aspherical surfaces incorporated into the indirect ophthalmoscopy lens. The first use of a double aspheric indirect ophthalmoscopy lens, designed for use with the indirect ophthalmoscope was described in U.S. Pat. No. 4,738,521, by David Volk, wherein a lens for use in indirect ophthalmoscopy had both the front and back surfaces of the lens being aspheric surfaces of revolution of conoid type. This double aspheric lens substantially improved the aerial image formed and reduced aberrations of the image including field curvature, astigmatism, and distortion. The use of double aspheric lenses, wherein the surfaces are particularly conoid surfaces, has been found to be of distinct advantage in indirect ophthalmoscopy and has made possible in use of much stronger lenses while providing increased clarity of the image with increased size of the field of view.
More recently, there has been developed a symmetrical double aspheric indirect ophthalmoscopy lens particularly suited for use with the slit lamp biomicroscope. This lens is described in U.S. Pat. No. 4,627,694, also by David Volk. The symmetrical double aspheric lenses as shown in this patent are of small diameter, with the aspheric surfaces described as having decreasing curvature from the apices of the surfaces peripheralward, and providing improved correction of aberrations including field curvature, astigmatism, and distortion. Lenses made according to this design have demonstrated themselves to be better suited for use with the slit lamp biomicroscope and have yielded significant improvement in the examiner's ability to see details in the aerial image of the fundus, yet the lens design does not account for pupil aberrations, which may be inherent in the lens design and which degrade the optical and performance characteristics of a lens, especially as it relates to observation of the fundus image using the slit lamp biomicroscope. Similarly, other prior indirect ophthalmoscopy lenses have apparently neglected completely the effects of pupil aberration in their design.
Particular problems arise when attempting to use a slit lamp biomicroscope for viewing of the aerial image formed by an indirect ophthalmoscopy lens. If the lens is of lower power, the beam of light from the slit lamp light source associated with the biomicroscope cannot be enlarged sufficiently to fill the full aperture of the lens, leaving a considerable portion of the lens unused in its condensing function. With the development of the double aspheric indirect ophthalmoscopy lenses described above, this problem was overcome by enabling the use of higher powered lenses, allowing greater illumination of the fundus and increased field of view. Although such lens design improvements have played an important role in present day eye fundus diagnostic and therapeutic techniques, especially with respect to diagnosis of diseases of the vitreous and retina, there has not been developed an indirect ophthalmoscopy lens particularly designed for use with a slit lamp biomicroscope, which optimally corrects for pupil aberrations as well as the more commonly considered aberrations, field curvature, astigmatism, and distortion and as such provides an extremely wide field of view and achieves very high resolution of the image even at peripheral portions thereof.
An indirect ophthalmoscopy lens for use with a slit lamp biomicroscope must also be positioned relative to the patient's eye, such that the conjugate focus of the slit lamp light source through the lens is at approximately the center of the entrance pupil of the patient's eye. The lens must thus be positioned a sufficient distance from the entrance pupil to form the conjugate focus of the slit lamp light source at the proper position for greatest illumination of the fundus. For higher powered lenses, the lens is positioned relatively close to the front of the patient's cornea, while the microscope of the slit lamp is positioned at a significant distance from the patient in order to allow observation of the formed aerial image of the fundus. The distance from the aerial image to the biomicroscope apparatus is dependent upon the attributes of the slit lamp microscope and particularly the focal distance of the objective lens system of the microscope. For the purpose of providing a wider field of view of the fundus by means of slit lamp ophthalmoscopy, the particular diameters of the more highly powered prior art lenses have been made relatively large, such that light rays originating at the more peripheral portions of the illuminated fundus, proceeding through the pupil and cornea, are incident upon the posterior lens surface at its periphery. Although refracted through the lens and contributing to the aerial image formation, these peripheral rays, as a result of inadequate lens design, in fact do not provide peripheral fundus imagery to the practitioner viewing through the slit lamp biomicroscope. This is due to the pupil aberrations of the indirect ophthalmoscopy lens, and the fact that the lens design has not addressed the optical characteristics and requirements of the slit lamp biomicroscope itself. It is therefore seen that the field of view and the image quality obtainable by prior art indirect ophthalmoscopy lenses has not been optimized for examination using a slit lamp microscope, the quality of the imagery of the eye pupil as it specifically relates to the slit lamp microscope pupil having been completely neglected. If the indirect ophthalmoscopy lens has significant pupil aberration, there will be excess vignetting of light rays, even at the mid-peripheral portions of the field of view. In certain cases, the rays from the edge of the field of view may completely miss the objective of the slit lamp microscope. Additionally, as previously stated, the illuminating system of the slit lamp biomicroscope is such that light from the slit lamp light source is reflected and converged to form a real aerial image of the slit lamp light source between the reflecting surface of the microscope and the indirect ophthalmoscopy lens. As the hand-held indirect ophthalmoscopy lens should be positioned in front of the patient's eye at a location which provides that the conjugate focus of the slit lamp source is at or near the center of the entrance pupil of the patient's eye, the slit lamp light source is relatively close to the indirect lens. The quality of the imagery of the eye pupil into the slit lamp microscope "pupil" will also be dependent upon such positioning, and again excess pupil aberrations may result in less than desired illumination of the eye fundus.